This case control study of the prevalence of colonization of the pneumococcus in HIV-infected children was begun on December, 1996 and is currently ongoing. This study will be significant in that it will define for the first time possible mechanisms behind the protective effect against blood infection with pneumococci of cotrimoxazole prophylaxis against an unrelated disease, PCP. It will also define whether cotrimoxazole prophylaxis influences the resistance pattern of penumococci, currently a major emerging pathogen as defined by the Centers for Disease Control. In addition, this study inaugurates the concept of "PCRU-on-wheels". The study is being conducted by PCRU staff in 4 clinic locations, with central processing and data collection in the PCRU.